Signal transduction mediated by heterotrimeric G-proteins occurs throughout nature and is best represented by phototransduction in retinal rods. However, how trimeric G-proteins subserve vertebrate vision outside photoreceptors is still poorly understood. Likewise, basic information in phototransduction, such as the mechanisms underlying sensitivity and kinetic differences between rods and cones, is missing. Mutations in visual G-protein pathways cause debilitating diseases such as congenital stationary night blindness, bradyopsia, and photoreceptor degeneration. This application is built on three novel preliminary findings: 1) rhodopsin kinase may act as a GTPase accelerating protein (GAP) to ensure timely recovery of phototransduction under dim light condition. 2) cone phototransduction recovery is rate-limited by cone transducin turn-off. 3) more than one type of G? subunits mediate depolarizing bipolar cell light responses. We propose here to use an integrated approach combining mouse genetics, biochemistry, genomics, cell biology, imaging, ERG, and single cell electrophysiology to study the mechanisms and physiological relevance of the aforementioned findings. Aim-1 will investigate mechanism and function of rhodopsin kinase mediated acceleration of phototransduction recovery to determine whether a higher level of rhodopsin kinase in cone help to ameliorate symptoms of human bradyopsia patients. Aim-2 will seek quantitative validation of the rate- limiting step of cone phototransduction recovery and explore the identity of the second slowest step of rod phototransduction recovery. Useful mouse strains and procedures will be developed to allow comparisons between rods and cones derived from the same animal. Aim-3 will examine the role of G3 and G?13 in retina to see whether their expression levels and interactions with G? subunit confer different light response properties to different types of depolarizing bipolar cells. By completing these aims we expect to know how trimeric G-proteins are used in vision beyond phototransduction and how rod and cone may differ in their sensitivity and speed of their responses to light. Many useful mouse lines will be made, characterized, preserved, and distributed here to facilitate future discovery. Finally, knowing the normal visual mechanism is indispensable for efficient prevention, preservation, and restoration of vision in patients suffering from various debilitating blinding diseases.